Field of the Invention
The present invention relates generally to gas turbine engine combustor fuel injectors and, more particularly, to fuel injector conduits having laminated fuel strips.
Fuel injectors, such as in gas turbine engines, direct pressurized fuel from a manifold to one or more combustion chambers. Fuel injectors also prepare the fuel for mixing with air prior to combustion. Each injector typically has an inlet fitting connected to the manifold, a tubular extension or stem connected at one end to the fitting, and one or more spray nozzles connected to the other end of the stem for directing the fuel into the combustion chamber. A fuel conduit or passage (e.g., a tube, pipe, or cylindrical passage) extends through the stem to supply the fuel from the inlet fitting to the nozzle. Appropriate valves and/or flow dividers can be provided to direct and control the flow of fuel through the nozzle. The fuel injectors are often placed in an evenly-spaced annular arrangement to dispense (spray) fuel in a uniform manner into the combustor chamber. An air cavity within the stem provides thermal insulation for the fuel conduit. A fuel conduit is needed that can be attached to a valve housing and to the nozzle. The fuel conduit should be tolerant of low cycle fatigue (LCF) stresses caused by stretching of the conduit which houses the conduit and which undergoes thermal growth more than the cold conduit. The attachment of the conduit to the valve housing should be a reliable joint which does not leak during engine operation. Fuel leaking into the hot air cavity can cause detonations and catastrophic over pressures.
A fuel injector typically includes one or more heat shields surrounding the portion of the stem and nozzle exposed to high temperature compressor discharge air. The heat shields are used for thermal insulation from the hot compressor discharge air during operation. This prevents the fuel from breaking down into solid deposits (i.e., xe2x80x9ccokingxe2x80x9d) which occurs when the wetted walls in a fuel passage exceed a maximum temperature (approximately 400 F. (200 C.) for typical jet fuel). The coke in the fuel nozzle can build up and restrict fuel flow through the fuel nozzle rendering the nozzle inefficient or unusable. One such heat shield assembly is shown in U.S. Pat. No. 5,598,696 and includes, a pair of U-shaped heat shield members secured together to form an enclosure for the stem portion of the fuel injector. At least one flexible clip member secures the heat shield members to the injector at about the midpoint of the injector stem. The upper end of the heat shield is sized to tightly receive an enlarged neck of the injector to prevent the compressor discharge air from flowing between the heat shield members and the stem. The clip member thermally isolates the heat shield members from the injector stem. The flexibility of the clip member permits thermal expansion between the heat shield members and the stem during thermal cycling, while minimizing the mechanical stresses at the attachment points.
Another stem and heat shield assembly is shown in U.S. Pat. No. 6,076,356 disclosing a fuel tube completely enclosed in the injector stem such that a stagnant air gap is provided around the tube. The fuel tube is fixedly attached at its inlet end and its outlet end to the inlet fitting nozzle, respectively, and includes a coiled or convoluted portion which absorbs the mechanical stresses generated by differences in thermal expansion of the internal nozzle component parts and the external nozzle component parts during combustion and shut-down. Many fuel tubes also require secondary seals (such as elastomeric seals) and/or sliding surfaces to properly seal the heat shield to the fuel tube during the extreme operating conditions occurring during thermal cycling. Such heat shield assemblies as described above require a number of components, and additional manufacturing and assembly steps, which can increase the overall cost of the injector, both in terms of original purchase as well as a continuing maintenance. In addition, the heat shield assemblies can take up valuable space in and around the combustion chamber, block air flow to the combustor, and add weight to the engine. This can all be undesirable with current industry demands requiring reduced cost, smaller injector size (xe2x80x9cenvelopexe2x80x9d) and reduced weight for more efficient operation.
More conventional nozzles employ primary and secondary nozzles in which only the primary nozzles are used during start-up. Both nozzles are used during higher power operation. The flow to the secondary nozzles is reduced or stopped during start-up and lower power operation. Fuel injectors having pilot and main nozzles have been developed for staged combustion. Primary and secondary nozzles discharge at approximately the same axial location in the combustor. Fuel injectors having main and pilot nozzles have been developed for more efficient and cleaner-burning, as the fuel flow can be more accurately controlled and the fuel spray more accurately directed for the particular combustor requirement. Fuel injectors having main and pilot nozzles use multiple fuel circuits discharging into different axial and radial locations in the combustion air flow field to provide good air and fuel mixing at high power. At low power some of the circuits are turned off to maintain a locally higher fuel/air ratio at the remaining fuel injection locations. The circuits and nozzles which are turned off at low power are referred to as main circuits and main nozzles. The circuits and nozzles which are left let on to keep the combustion flame from extinguishing are referred to as pilot circuits and pilot nozzles. The pilot and main nozzles can be contained within the same nozzle stem assembly or can be supported in separate nozzle assemblies. Dual nozzle fuel injectors can also be constructed to allow further control of the fuel for dual combustors, providing even greater fuel efficiency and reduction of harmful emissions.
A typical technique for routing fuel through the stem portion of the fuel injector is to provide a fuel conduit having concentric passages within the stem, with the fuel being routed separately through different passages. The fuel is then directed through passages and/or annular channels in the nozzle portion of the injector to the spray orifice(s). U.S. Pat. No. 5,413,178, for example, discloses concentric passages where the pilot fuel stream is routed down and back along the main nozzle for cooling purposes. This can also require a number of components and additional manufacturing and assembly steps, which can all be contrary to desirable cost and weight reduction and small injector envelope.
U.S. Pat. No. 6,321,541 addresses these concerns and drawbacks with a fuel injector that includes an inlet fitting, a stem connected at one end to the inlet fitting, and one or more nozzle assemblies connected to the other end of the stem and supported at or within the combustion chamber of the engine. A fuel conduit in the form of a single elongated laminated feed strip extends through the stem to the nozzle assemblies to supply fuel from the inlet fitting to the nozzle(s) in the nozzle assemblies. An upstream end of the feed strip is directly attached (such as by brazing or welding) to the inlet fitting without additional sealing components (such as elastomeric seals). A downstream end of the feed strip is connected in a unitary (one piece) manner to the nozzle. The single feed strip has convolutions along its length to provide increased relative displacement flexibility along the axis of the stem and reduce stresses caused by differential thermal expansion due to the extreme temperatures the nozzle is exposed to. This reduces or eliminates a need for additional heat shielding of the stem portion of the injector.
The laminate feed strip and nozzle are formed from a plurality of plates. Each plate includes an elongated, feed strip portion and a unitary head (nozzle) portion, substantially perpendicular to the feed strip portion. Fuel passages and openings in the plates are formed by selectively etching the surfaces of the plates. The plates are then arranged in surface-to-surface contact with each other and fixed together such as by brazing or diffusion bonding, to form an integral structure. Selectively etching the plates allows multiple fuel circuits, single or multiple nozzle assemblies and cooling circuits to be easily provided in the injector. The etching process also allows multiple fuel paths and cooling circuits to be created in a relatively small cross-section, thereby, reducing the size of the injector.
The feed strip portion of the plate assembly is mechanically formed such as by bending to provide the convoluted form. In one embodiment, the plates all have a T-shape in plan view. In this form, the head portions of the plate assembly can be mechanically formed into a cylinder having an annular cross-section, or other appropriate shape. The ends of the head can be spaced apart from one another or can be brought together and joined, such as by brazing or welding. Spray orifices are provided on the radially outer surface, radially inner surface and/or ends of the cylindrical nozzle to direct fuel radially outward, radially inward and/or axially from the nozzle.
It is desirable to have a fuel conduit that is more flexible, has less bending stress and, is therefore, less susceptible to low cycle fatigue than previous feed strip designs. It is also desirable to have a feed strip with good relative displacement flexibility along the axis of the stem and that reduce stresses caused by differential thermal expansion due to the extreme temperatures to which the nozzle is exposed. It is also desirable to have a feed strip that provides a smaller envelope for the heat shield which, in turn, has a small circumferential width in the flow and lower drag and associated flow losses making for a more aerodynamically efficient design.
A fuel injector conduit includes a single feed strip having a single bonded together pair of lengthwise extending plates. Each of the plates has a single row of widthwise spaced apart and lengthwise extending parallel grooves. The plates are bonded together such that opposing grooves in each of the plates are aligned forming internal fuel flow passages through the length of the strip from an inlet end to an outlet end.
The feed strip includes a radially extending substantially straight middle portion between the inlet end and the outlet end. A straight header of the fuel injector conduit extends transversely (in an axially aftwardly direction) away from the outlet end of the middle portion and leads to an annular main nozzle. Radial thermal growth of the feed strip is accommodated by deflection of bending arms of the strip that are fully or partially transverse to or deflect substantially transversely to the middle portion. The straight header is a first bending arm A1 and it is the longest of the bending arms.
In the exemplary embodiment of the invention, the middle portion is slightly bowed and has a radius of curvature greater than a length of the middle portion. The middle portion is slightly bowed for ease of installation.
In the exemplary embodiment of the invention, the feed strip has at least one acute bend between the inlet end and the middle portion and a bend between the outlet end and the middle portion. The acute bend has radially inner and outer arms, respectively having second and third bending arm lengths. The inner and outer arms are angularly spaced apart by an acute angle. The second and third bending arm lengths are fully or partially transverse to or deflect substantially transversely to the middle portion. The feed strip has fuel inlet holes in the inlet end connected to the internal fuel flow passages. The inlet end is fixed within a valve housing.
In a further embodiment of the invention, the annular main nozzle is fluidly connected to the outlet end of the feed strip and integrally formed with the feed strip from the single bonded together pair of lengthwise extending plates. The internal fuel flow passages extend through the feed strip and the annular main nozzle. Annular legs extend circumferentially from at least a first one of the internal fuel flow passages through the main nozzle. Spray orifices extend from the annular legs through at least one of the plates. The annular legs may have waves. The annular legs may include clockwise and counterclockwise extending annular legs. The clockwise and counterclockwise extending annular legs may have parallel first and second waves, respectively, and the spray orifices may be located in alternating ones of the first and second waves so as to be substantially aligned along a circle.
In a more detailed embodiment, the conduit includes a pilot nozzle circuit which includes clockwise and counterclockwise extending pilot legs extending circumferentially from at least a second one of the internal fuel flow passages through the main nozzle.
The invention includes a fuel injector including an upper valve housing, a hollow stem depending from the housing, at least one fuel nozzle assembly supported by the stem, and the fuel injector conduit extending between the housing through the stem to the nozzle assembly. The injector may further include a main mixer having an annular main housing with openings aligned with the spray orifices. An annular cavity is defined within the main housing and the main nozzle is supported by the main housing within the annular cavity. An annular slip joint seal is disposed in each set of the openings aligned with each one of the spray orifices. The housing may include inner and outer heat shields and the inner heat shield may further include inner and outer walls and an annular gap therebetween such that the openings pass through the inner and outer heat shields. The annular slip joint seal may be attached to the inner wall of the inner heat shield.
The invention also provides a fuel injector having an annular main nozzle, a main mixer having an annular main housing with openings aligned with spray orifices in a main nozzle, and an annular cavity defined within the main housing. The main nozzle is received within the annular cavity and an annular slip joint seal is disposed in each set of the openings aligned with each one of the spray orifices. The housing may further include inner and outer heat shields, respectively, and the inner heat shield may include inner and outer walls with an annular gap therebetween. The openings may pass through the inner and outer heat shields, 196) and the annular slip joint seal may be attached to the inner wall of the inner heat shield.
The feed strip of the present invention has good relative displacement flexibility along the axis of the stem and low stresses caused by differential thermal expansion due to the extreme temperatures to which the nozzle is exposed. The present invention provides for a fuel conduit that allows the use of a smaller envelope for hollow stem which serves as a heat shield for the conduit. The hollow stem, in turn, has a small circumferential width in the flow and, therefore, lowers drag and associated flow losses making for a more aerodynamically efficient design.